An electromagnetic impedance sensor for detecting a detection object by using a coil, i.e., a search coil. Specifically, when the detection object is made of conductor or soft magnetic material, and the detection object approaches the search coil, an impedance of the search coil changes, so that the detection object is detected based on impedance change. This sensor is disclosed in, for example, JP-A-H09-175319.
The conventional sensor has a difficulty that detection sensitivity may disperse with reference to a position of the detection object in a detection space when the detection space is comparatively large in view of dimensions of the detection object. Here, the sensor can detect the object in the detection space.
This is because the detection sensitivity fluctuates with reference to a relationship between the position of the detection object in the space and the coil arrangement of the search coil. The search coil is arranged in the detection space to face the detection space in order to cover the detection space. Specifically, since magnetic flux at each portion of the detection space may disperse along with a direction in parallel to a coil surface of the coil, the detection sensitivity fluctuates.
Thus, it is required for the sensor to have high detection accuracy.